Distributing OpenPGP Keys with Signed Keylist Subscriptions
First Look Media
ietf@sendmiles.email
https://rmrm.io
The Intercept
micah.lee@theintercept.com
https://micahflee.com/
First Look Media
nat.welch@firstlook.media
https://natwelch.com
Security
OpenPGP
GPGSync
GPG
Keylist
This document specifies a system by which an OpenPGP client may
subscribe to an organization's keylist to keep its internal
keystore up-to-date. Ensuring that all members of an
organization have their colleagues' most recent PGP public keys
is critical to maintaining operational security. Without the
most recent keys and a source of trust for those keys (as this
document specifies), users must manually update and sign each
others keys -- a system that is untenable in larger
organizations. This document proposes a experimental format for
the keylist file as well as requirements for clients who wish to
implement this experimental keylist subscription functionality.
This document specifies a system by which clients may subscribe
to cryptographically signed keylists. This system allows for
seamless key rotation across entire organizations and enhances
operational security. To enable cross-client compatibility, this
document provides a experimental format for the keylist, its
cryptographic verification, and the method by which it is
retreived by the client. The user interface by which a client
provides this functionality to the user is out of scope, as is
the process by which the client retrieves public keys. Other
non-security-related implementation details are also out of
scope.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in
.
This document uses the terms "OpenPGP", "public key",
"private key", "signature", and "fingerprint" as defined by
OpenPGP Message Format
.
The term "keylist" is defined as a list of OpenPGP public
key fingerprints and accessible via a URI. The exact format
of this data is specified in
.
An "authority key" is defined as the OpenPGP secret key used
to sign a particular keylist. Every keylist has a
corresponding authority key, and every authority key has at
least one corresponding keylist. A single authority key
SHOULD NOT be used to sign multiple keylists.
To be "subscribed" to a keylist means that a program will
retreive that keylist on a regular interval. After
retrieval, that program will perform an update to an
internal OpenPGP keystore.
A "client" is a program that allows the user to subscribe to
keylists. A client may be an OpenPGP client itself or a
separate program that interfaces with an OpenPGP client to
update its keystore.
RFC Editor: please remove this section prior to
publication.Development of this Internet draft takes place on GitHub at
Keylist-RFC.
A mailing list is available for discussion at Keylists
mailing list.
As new keys are created and other keys are revoked, it is
critical that all members of an organization have the most
recent set of keys available on their computers. Keylists enable
organizations to publish a directory of OpenPGP keys that
clients can use to keep their internal keystores up-to-date.
A single client may subscribe to any number of keylists.
When a client first subscribes to a keylist, it SHOULD
update or import every key present in the keylist into its
local keystore. Keylist subscriptions SHOULD be persistent
--that is, they should be permanently stored by the client
to enable future automatic updates.
To subscribe to a keylist, the client must be aware of the
keylist URI (see ), and the
fingerprint of the authority key used to sign the
keylist. The protocol used to retrieve the keylist and its
signature SHOULD be HTTPS (see ),
however other implementation MAY be supported. A client
implementing keylist functionality MUST support the
retrieval of keylists and signatures over HTTPS. All other
protocols are OPTIONAL.
A client MUST NOT employ a trust-on-first-use model for
determining the fingerprint of the authority key; it must be
explicitly provided by the user.
The process by which the client stores its keylist
subscriptions is out of scope, as is the means by which
subscription functionality is exposed to the end-user.
The primary purpose of keylists is to enable periodic
updates of OpenPGP clients' internal keystores. We RECOMMEND
that clients provide a default refresh interval of less than
one day, however we also RECOMMEND that clients allow the
user to select this interval. The exact time at which
updates are performed is not critical.
To perform an update, the client MUST perform the following
steps on each keylist to which it is subscribed. The steps
SHOULD be performed in the given order.
Obtain a current copy of the keylist from its URI.
Obtain a current copy of the keylist's signature
data from its URI, which is included in the keylist data
format specified in .
Using the keylist and the keylist's signature,
cryptographically verify that the keylist was signed
using the authority key. If the signature does not
verify, the client MUST abort the update of this
keylist and SHOULD alert the user. The client SHOULD
NOT abort the update of other keylists to which it
is subscribed, unless they too fail signature
verification.
Validate the format of the keylist according to
. If the keylist is in an invalid format, the client
MUST abort the update this keylist and SHOULD alert
the user.
For each fingerprint listed in the keyfile, if a
copy of the associated public key is not present in
the client's local keystore, retrieve it from the
keyserver specified by the keylist (see
) or, if the keylist specifies
no keyserver, from any keyserver.
If the key is already present and not revoked,
refresh it from a keyserver. If it is present and
revoked, do nothing.
To ensure authenticity of a keylist during an update, the
client MUST verify that the keylist's data matches its
cryptographic signature, and that the public key used to
verify the signature matches the authority key fingerprint
given by the user.
For enhanced security, it is RECOMMENDED that keylist
operators sign each public key listed in their keylist with
the authority private key. This way, an organization can
have an internal trust relationship without requiring
members of the organization to certify each other's public
keys.
The following are format specifications for the keylist file and
its signature file.
The keylist MUST be a valid JavaScript Object Notation
(JSON) Data Interchange Format
object with specific keys
and values, as defined below. Note that unless otherwise specified,
'key' in this section refers to JSON keys--not OpenPGP keys.
To encode metadata, the keylist MUST have a "metadata" root key
with an object as the value ("metadata object").
The metadata object MUST contain a "signature_uri" key whose value
is the URI string of the keylist's signature file. All metadata keys
apart from "signature_uri" are OPTIONAL.
The metadata object MAY contain a "keyserver" key with the value of the
URI string of the keyserver from which the OpenPGP keys in the keylist
should be retrieved.
The metadata object MAY contain a "comment" key with the
value of any string. The metadata object MAY also contain other arbitrary
key-value pairs.
The keylist MUST have a "keys" key with an array as the value.
This array contains a list of OpenPGP key fingerprints and
metadata about them. Each item in the array MUST be an object.
Each of these objects MUST have a "fingerprint" key with the
value of a string that contains the full 40-character
hexadecimal public key fingerprint, as defined in OpenPGP
Message Format
. Any number of space characters (' ', U+0020) MAY be included
at any location in the fingerprint string. These objects MAY
contain "name", "email", and "comment" key-value pairs, as well
as any other key-value pairs relevant.
The following is an example of a valid keylist.
{
"metadata": {
"signature_uri": "https://www.example.com/keylist.json.asc"
"comment": "This is an example of a keylist file"
},
"keys": [
{
"fingerprint": "927F419D7EC82C2F149C1BD1403C2657CD994F73",
"name": "Micah Lee",
"email": "micah.lee@theintercept.com",
"comment": "Each key can have a comment"
},
{
"fingerprint": "1326CB162C6921BF085F8459F3C78280DDBF52A1",
"name": "R. Miles McCain",
"email": "0@rmrm.io"
},
{
"fingerprint": "E0BE0804CF04A65C1FC64CC4CAD802E066046C02",
"name": "Nat Welch",
"email": "nat.welch@firstlook.org"
}
]
}
The signature file MUST be an ASCII-armored 'detached
signature' of the keylist file, as defined in OpenPGP
Message Format
.
GPG Sync, an open source program created by one of the authors,
implements this experimental standard. GPG Sync is used by First
Look Media and the Freedom of the Press Foundation to keep
OpenPGP keys in sync across their organizations, as well as to
publish their employee's OpenPGP keys to the world. These
organizations collectively employ more than 200 people and have
used the system described in this document successfully for
multiple years.
GPG Sync's existing code can be found at
<https://github.com/firstlookmedia/gpgsync>
First Look Media's keylist file can be found at
<https://github.com/firstlookmedia/gpgsync-firstlook-fingerprints>
The keylist subscription functionality defined in this
document provide a number of security benefits, including:
The ability for new keys to be quickly distributed
across an organization.
It removes the complexity of key distribution from
end users, allowing them to focus on the content of
their communications rather than on key management.
The ability for an organization to prevent the
spread of falsely attributed keys by centralizing
the public key discovery process within their
organization.
There is a situation in which keylist subscriptions could
pose a potential security threat. If the both the authority
key and the keylist distribution system were to be
compromised, it would be possible for an attacker to
distribute false keys. We believe, however, that the
security benefits of this system strongly outweigh the
drawbacks.
This document has no actions for IANA.